SiC complex sintered bodies and production thereof

ABSTRACT

A silicon carbide complex sintered body including 20 to 99 wt % of SiC, 80 to 0.5 wt % of at least one compound selected from the group consisting of W 2  B 5  and/or MoB 2 , and 0.5 to 5 wt % of at least one material selected from the group consisting of boron, carbon, and boron carbide. A process for producing the sintered body by preparing a mixed powder is also disclosed, wherein the mixed powder includes 20 to 99 wt % of SiC powder having an average particle diameter of not greater than 5 microns, 80 to 0.5 wt % of at least one compound selected from the group consisting of W 2  B 5  and MoB 2 , and 0.1 to 5 wt %, when calculated as boron, of boron or a compound containing boron, and 0.1 to 5 wt %, when calculated as carbon, molding the mixture, and firing the molding in a temperature range from 1,900° to 2,500° C. in vacuum or in an inert atmosphere.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to SiC complex sintered bodies having excellent high temperature-characteristics unique to SiC itself and high toughness. The sintered bodies can be produced by pressureless sintering. The present invention also relates to a process for producing such SiC complex sintered bodies.

2. Related Art Statement

As SiC sintered bodies having a single phase, there are silicon carbide sintered bodies containing a B-C based sintering assistant (hereinafter referred to as "B-C based SiC") and silicon carbide sintered bodies containing an Al based sintering assistant (hereinafter referred to as "Al based SiC"). Although the B-C based SiC has excellent characteristics at high temperatures, its toughness is poor (K_(IC) =2-3 MN/m^(3/2)). On the other hand, although the Al based SiC has excellent toughness, its characteristics at high temperatures are poor.

With respect to Al based SiC, Japanese patent application Laid-open No. 60-186,468 discloses ceramic structural bodies and a process for producing the same, aiming at higher toughness. The ceramic structural bodies contain at least one kind of boride of elements in Group Va or Via of the periodic in silicon carbide. The sintered bodies are obtained by mixing elements in a specified composition, molding the mixture, and firing the molding in a temperature range from 1,900° C. to 2,500° C.

However, according to specific examples of ceramic structural bodies disclosed in the above Japanese patent application Laid-open No. 60-186,468, fracture toughness is improved by using W₂ B or MoB₂ and Al as an additive and a sintering assistant, respectively, and promoting densification by hot pressing. The reason why the firing is effected by hot pressing is that since W₂ B is difficult to sinter, densification cannot be attained by pressureless sintering. When MoB₂ is added to Al based SiC, densification cannot be effected by pressureless sintering either. However, sintered bodies having complicated shapes cannot be obtained by hot pressing, which has low industrial applicability from standpoint of the mass productivity and manufacturing cost.

When an Al based assistant is used, an intergranular phase having a low melting point generally remains in an SiC intergranular phase, so that strength and oxidation resistance at high temperatures are. Consequently, the thus obtained sintered bodies have fatal defects as materials for use at high temperatures.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the above-mentioned problems, and to provide high strength, high toughness SiC complex sintered bodies and a process for producing such highly tough SiC complex sintered bodies by pressureless sintering, while maintaining the characteristics at high temperatures which are unique to SiC.

According to a first aspect of the present invention, there is a provision of an SiC complex sintered body characterized by consisting essentially of 20 to 99 wt% of SiC, 80 to 0.5 wt% of W₂ B₅ and/or MoB₂, and 0.5 to 5% of at least one kind of boron, carbon, and boron carbide.

According to a second aspect of the present invention, there is a provision of a process for producing an SiC complex sintered body, comprising the steps of mixing powders of a mixture consisting essentially of 20 to 99 wt% of SiC powder having an average particle diameter of not more than 5 μm, 80 to 0.5 wt%, when calculated as W₂ B₅ and/or MoB₂, of W₂ B₅ or a compound producing W₂ B₅ and/or MoB₂ or a compound producing MoB₂, 0.1 to 5 wt%, when calculated as boron, of boron or a compound containing boron, and 0.1 to 5 wt%, when calculated as carbon; of carbon or an organic compound producing carbon, molding the mixture; and firing the molding in a temperature range from 1,900° C. to 2,500° C. in vacuum or an inert atmosphere.

In the above construction, by incorporating a specified amount of W₂ B₅ and/or MoB₂ into B-C based SiC, toughness which is poor for the B-C based SiC can be improved and the SiC complex sintered bodies having high strength at high temperatures and high toughness can be obtained by pressureless firing. That is, since W₂ B₅ and MoB₂ have high thermal stability, they exhibit excellent characteristics at high temperatures. Further, since W₂ B₅ has a higher Young's modulus as compared with SiC, crack deflection phenomenon that cracks proceeding in an SiC matrix are recoiled by W₂ B₅ grains is effectively exhibited to improve toughness. On the other hand, since hardness greatly differs between MoB₂ and SiC, the crack deflection phenomenon that cracks proceeding in a sintered body are recoiled at interfaces between MoB₂ grains and SiC grains is effectively exhibited to also improve toughness. Further, W₂ B₅ itself also has an effect to promote densification during the sintering, and thus high density which has not been attained in conventional SiC sintered bodies can be obtained by pressureless sintering.

In addition, in the B-C based SiC, it is known that SiC grains abnormally grow up to several hundreds μm during a final stage of the sintering, i.e., at sintering temperatures not less than 2,100° C. to hinder densification and greatly deteriorate characteristics of sintered bodies. Thus, it was necessary to set the sintering temperature at substantially not more than 2,100° C. Since MoB₂ can effectively restrain the above abnormal grain growth, sintering can be effected at sintering temperatures more than 2,100° C. Consequently, high density which has not been attained in conventional SiC sintered bodies can be attained by pressureless sintering and sintered bodies having desired shapes can be produced.

These and other objects, features and advantages of the invention will be appreciated when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains without departing from the spirit of the invention or the scope of claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is made to the attached drawings, wherein:

FIG. 1 is a flow chart illustrating an example of producing steps of the producing process according to the present invention;

FIG. 2 is a diffraction pattern in an X-ray diffraction test using a CuKα line with respect to a sintered body in Example 4 obtained according to the present invention; and

FIG. 3 is a diffraction pattern in an X-ray diffraction test using a CuKα line with respect to a sintered body in Example 103 obtained according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

First, Young's modulus, hardnesses, and melting points of various bodies and SiC are shown together in the following Table 1. These data were extracted from Samsonoff: High Melting Point Compounds Manual and El Be Caterinicoff: Super High Melting Point Materials Manual.

                  TABLE 1                                                          ______________________________________                                         Young's                      Melting                                           modulus           Hardness   point                                             (kg/mm.sup.2)     (kg/mm.sup.2)                                                                             (°C.)                                      ______________________________________                                         W.sub.2 B.sub.5                                                                        79,000        --         2,370                                         TiB.sub.2                                                                              54,000        3,370 ± 60                                                                             2,790                                         ZrB.sub.2                                                                              35,000        2,252 ± 22                                                                             3,200                                         NbB.sub.2                                                                              65,000        2,600      3,000                                         Mo.sub.2 B.sub.5                                                                       --            1,200      2,350                                         HfB.sub.2                                                                              --             2,900 ± 500                                                                           3,250                                         TaB.sub.2                                                                              26,200        2,500 ± 42                                                                             3,037                                         SiC     39,400        3,600      2,540                                         ______________________________________                                    

The above-mentioned effects are first obtained in B-C based complex sintered bodies of SiC-W₂ B₅ and/or MoB₂, but have not formerly been known at all.

Now, limitations made in the present invention will specifically be explained.

The reason why the average particle diameter of SiC powder as a starting material is limited to not more than 5 μm is that if it exceeds 5 μm, densification cannot be effected by pressureless sintering.

The reason why the mixing ratio of SiC is limited to a range from 20 to 99 wt% is that if the total amount of the other ingredients exceeds 80 wt%, characteristics of SiC as a base cannot fully be exhibited, while if it is less than 1 wt%, effects of improving characteristics due to addition of the additives cannot be obtained. The mixing ratio of SiC ranges preferably from 30 to 95 wt%, more preferably from 40 to 90 wt%.

The reason why the total mixing ratio of W₂ B₅ and/or MoB₂ is limited to a range from 80 to 0.5 wt% is that if it exceeds 80 wt%, characteristics at high temperatures are deteriorated, while if it is less than 0.5 wt%, toughness is not improved and the effect of restraining abnormal grain growth cannot be obtained. As to W₂ B₅ alone, the mixing ratio is preferably from 70 to 5 wt%, more preferably from 60 to 10 wt%. As to MoB₂ alone, the mixing ratio is preferably from 70 to 0.5 wt%, and more preferably from 60 to 0.5 wt%.

The reason why the total amount of at least one kind of boron, carbon and boron carbide in the sintered body is limited to a range from 0.5 to 5 wt% is that if it falls outside the range, densification is not fully effected so that high strength SiC complex sintered bodies cannot be obtained.

The reason why an amount of B or the B-forming compound is limited to 0.1 to 5 wt% is that if it is less than 0.1 wt%, an effect due to addition thereof cannot be recognized and densification is not caused, while if it exceeds 5 wt%, densification is hindered and a great amount of B remains in the intergranular phase to deteriorate high temperature characteristics.

Further, the reason why an amount of C or the C-forming compound is limited to 0.1 to 5 wt% is that if it is less than 0.1 wt%, an SiO₂ film on the surface of SiC cannot fully be removed, while if it is exceeds 5 wt%, a great amount of free carbon remains in the sintered body to deteriorate the characteristics.

Next, the process for producing SiC complex sintered bodies according to the present invention will be explained by way of example with reference to FIG. 1.

First, a starting powder of SiC is adjusted to the average particle diameter of not less than 5 μm, and W₂ B₅ or a W₂ B₅ -forming compound and/or MoB₂ or a MoB₂ -forming compound, B or a B-forming compound, and C or a C-forming compound are prepared as additives. The SiC starting powder and the additives are mixed and milled at specified proportions together with an appropriate binder such as isopropyl alcohol by using a wet type ball mill. After being dried, the milled mixture is granulated, and preliminarily molded. The preliminary molding is molded in a desired shape by hydrostatic press. The molding is finally fired in a temperature range from 1,900° C. to 2,500° C. in vacuum or in an inert gas atmosphere to obtain a sintered product. The temperature range is preferably from 1,900° to 2,300° C. when W₂ B₅ is employed alone.

The reason why the firing temperature is limited to the range from 1,900° to 2,500° C. is that if it is less than 1,900° C., densification cannot fully be effected, while if it is more than 2,500° C. silicon carbide is greatly decomposed to roughen the surface of the sintered body, so that sufficiently smooth surface cannot be obtained.

For example, the SiC starting powder to be used in the producing process according to the present invention consists of 93 wt% of β-SiC and the remainder being α-SiC, and has the average particle diameter of 0.42 μm and a specific surface area of 20.0 m² /g with the following composition.

                  TABLE 2                                                          ______________________________________                                                       (wt %)                                                           ______________________________________                                                Total Si     69.13                                                             Free  SiO.sub.2                                                                             0.47                                                              Free  Si     0.010                                                             Free  C      0.51                                                                    O      0.90                                                                    Al     0.056                                                                   Fe     0.060                                                                   Ca     0.016                                                                   Mg     0.001                                                                   K      <0.001                                                                  Na     0.001                                                                   Cu     --                                                                      Ti     0.007                                                                   N      0.27                                                       ______________________________________                                    

In the above producing process of the present invention, it may be that instead of addition of W₂ B₅ and/or MoB₂, W and/or Mo, or another boride or an oxide thereof is added separately from a boron-containing additive, and that W₂ B₅ and/or MoB₂ is formed in a molding or a mixed powder during an initial stage of firing or by another heat treatment before the firing. In this case, it is necessary to add an excess amount of B enough to fully synthesize W₂ B₅ and/or MoB₂. In addition, it is also possible to improve characteristics of sintered bodies by further densifying them in capsulized HIP or capsule-free HIP after the firing.

As the W₂ B₅ -forming material, W may be used. As the MoB₂ -forming material, Mo may be used. As the B-containing compound, B₄ C may be used. As the C-forming organic compound, phenolic resin may be used.

Next, the present invention will be explained in more detail with reference to specific experiments.

EXPERIMENT 1

First, a starting powder of SiC was prepared to have the average particle diameter of not more than 5 μm, which was mixed with W₂ B₅ as an additive and B (metallic boron) and C (carbon black) as sintering assistant in a wet type ball mill at a compounding recipe shown in Table 3 by using isopropyl alcohol. The mixture was dried and granulated, and then preliminarily molded and hydrostatically pressed at 3 tons/cm² to obtain a square plate of 60×60×6 mm. The thus obtained plate was heated up to 1,500° C. in vacuum, and then fired at 2,100° C. in argon atmosphere for one hour. In such a way, sintered bodies in the Examples according to the present invention and Comparative Examples were obtained. Example 5-2 is an example obtained by HIP treatment of a sintered body in Example 5 at 2,000° C. under 2,000 atm.

The thus obtained sintered bodies were mirror polished, and their denseness was evaluated by measuring relative densities based on a pore distribution. Further, strengths at room temperature and at high temperatures were measured at room temperature and 1,400° C. by a four point bending test according to JIS R-1601 (fine ceramic bending strength testing method). Moreover, toughness was evaluated by determining K_(IC) at room temperature according to Chevron Notch method, and W compounds were identified in the sintered bodies by the X ray diffraction test using a CuKα line. Results are shown in Table 3.

                                      TABLE 3(a)                                   __________________________________________________________________________                              Four point                                                        Sintering    bending strength                                             Additive                                                                            assistant                                                                           Relative density                                                                       (MPa)                                                        (wt %)                                                                              (wt %)                                                                              of sintered body                                                                       room        K.sub.IC                                                                             W compound in                              W.sub.2 B.sub.5                                                                     B  C (%)     temperature                                                                           1,400° C.                                                                    (MN/m.sup.3/2)                                                                       sintered body                       __________________________________________________________________________     Example 1                                                                             0.5  1.0                                                                               4.0                                                                              97      600    580  3.3   W.sub.2 B.sub.5                     Example 2                                                                             5    1.0                                                                               4.0                                                                              98      670    640  4.2   "                                   Example 3                                                                             10   1.0                                                                               4.0                                                                              98      720    700  4.6   "                                   Example 4                                                                             30   1.0                                                                               4.0                                                                              98      800    780  5.5   "                                   Example 5                                                                             50   1.0                                                                               4.0                                                                              97      820    800  5.9   "                                   Example 5-2                                                                           50   1.0                                                                               4.0                                                                              not less than 99                                                                       930    910  6.2   "                                   Example 6                                                                             70   1.0                                                                               4.0                                                                              94      770    690  5.5   "                                   Example 7                                                                             50   0.3                                                                               4.0                                                                              97      830    790  5.8   "                                   Example 8                                                                             50   2  4.0                                                                              96      780    750  5.8   "                                   Example 9                                                                             50   5  4.0                                                                              94      750    710  5.6   "                                   Example 10                                                                            50   1.0                                                                               2 97      810    800  5.9   "                                   __________________________________________________________________________

                                      TABLE 3(b)                                   __________________________________________________________________________                               Four point                                                       Sintering     bending strength                                            Additive                                                                            assistant                                                                            Relative density                                                                       (MPa)                                                       (wt %)                                                                              (wt %)                                                                               of sintered body                                                                       room        K.sub.IC                                                                             W compound in                             W.sub.2 B.sub.5                                                                     B  C  (%)     temperature                                                                           1,400° C.                                                                    (MN/m.sup.3/2)                                                                       sintered body                      __________________________________________________________________________     Comparative                                                                            0   1.0                                                                               4.0                                                                               96      550    530  2.6   not contained                      Example 1                                                                      Comparative                                                                           90   1.0                                                                               4.0                                                                               87      650    330  4.8   W.sub.2 B.sub.5                    Example 2                                                                      Comparative                                                                           50   0  4.0                                                                               82      420    330  --    "                                  Example 3                                                                      Comparative                                                                           50   10 4.0                                                                               87      600    520  4.9   "                                  Example 4                                                                      Comparative                                                                           50   1.0                                                                               0  64      280    220  --    "                                  Example 5                                                                      Comparative                                                                           50   1.0                                                                               10 86      490    480  5.2   "                                  Example 6                                                                      __________________________________________________________________________

As is evident from Table 3, Examples 1-10 meeting the composition range according to the present invention had higher density and more excellent strength at high temperatures as compared with Comparative Examples 1-6. Further, as compared with K_(IC) (2-3) in conventional SiC containing no W₂ B₅, Examples 1-10 exhibited higher K_(IC) values and their toughness increase. Example 5-2 having undergone HIP treatment after firing had excellent properties in all respects.

It was confirmed that the compound of W in the sintered body was W₂ B₅ shown in JCPDS card No. 30-1358.

A diffraction pattern of Example 4 in an X-ray diffraction test by using a CuKα line is shown in FIG. 2.

By the chemical analysis of the sintered bodies, 0.01 to 0.06 wt% of inevitable metallic impurities (Al, Fe, Ca, Mg, Ti, Mn, etc.) contained in the SiC starting material and the additives was detected.

EXPERIMENT 2

A boride other than W₂ B₅, a carbide or an oxide of W, or W alone was added, instead of W₂ B₅ as the indispensable ingredient in the present invention, to a SiC starting powder, which was milled and kneaded, and sintered bodies were obtained in the same process as in Experiment 1.

Then, the relative density of the sintered bodies and a compound of W were determined by the same methods as in Experiment 1. Results are shown in Table 4.

                  TABLE 4                                                          ______________________________________                                                                Relative                                                              Sintering                                                                               density                                                               assistant                                                                               of                                                             Additive                                                                              (wt %)   sintered W compound in                                         (wt %) B      C     body (%)                                                                              sintered body                                ______________________________________                                         Example 11                                                                              W:50     10     4.0 97     W.sub.2 B.sub.5                            Example 12                                                                              WC:50    10     4.0 95     W.sub.2 B.sub.5                            Comparative                                                                             W:50     1.0    4.0 60     WC, WB                                     Example 7                                                                      Comparative                                                                             W.sub.2 C:50                                                                            1.0    4.0 60     WC, WB                                     Example 8                                                                      Comparative                                                                             WC:50    1.0    4.0 59     WC, WB                                     Example 9                                                                      Comparative                                                                             W.sub.2 B:50                                                                            1.0    4.0 61     WB                                         Example 10                                                                     Comparative                                                                             WB:50    1.0    4.0 67     W.sub.2 B.sub.5, WB                        Example 11                                                                     Comparative                                                                             WO.sub.3 :50                                                                            1.0    4.0 57     WC, WB                                     Example 12                                                                     Comparative                                                                             W:50     5      4.0 64     WC, WB                                     Example 13                                                                     Comparative                                                                             WC:50    5      4.0 62     WC, WB                                     Example 14                                                                     ______________________________________                                    

As is evident from results in Table 4, in Comparative Examples 7-12 having an additive other than W₂ B₅, high relative density, i.e., densification of sintered bodies having a conventionally known amount of B (1 wt% added) could not be achieved. Further, as is evident from Comparative Examples 13 and 14, even if an amount of B was increased but it was not converted to W₂ B₅ in a sintered body, high relative density could not be achieved, either.

As is evident form Examples 11 and 12, when the amount of B was further increased and B was added more than 2.5 times in mole W (W: 50 wt%, B: 7.3 wt%), W₂ B₅ was formed in the sintered body during the firing initial stage, so that densification was effected as in the case that W₂ B₅ was added as shown in Example 1 of the captioned application.

As is clear in the aforegoing, according to the SiC complex sintered bodies and the producing process thereof in the present invention, toughness which is poor for the B-C based SiC can be improved by incorporating a specified amount of W₂ B₅ into the B-C based SiC while maintaining high temperature characteristics. Further, SiC complex sintered bodies having high strength at high temperatures and high toughness can be obtained.

In Table 5 is shown by comparison characteristics of SiC complex sintered bodies according to the present invention and a prior art technique.

                                      TABLE 5                                      __________________________________________________________________________                     Al based                                                                              B--C base                                                                             Al base                                                   SiC--W.sub.2 B.sub.5                                                                  sintering                                                                             sintering                                                                             sintering                                                 according to                                                                          assistant +                                                                           assistant +                                                                           assistant +                                               the invention                                                                         SiC--W.sub.2 B.sub.5                                                                  SiC    SiC                                              __________________________________________________________________________     High temper-                                                                            good   bad    good   bad                                              ature charac-                                                                  teristic                                                                       Toughness                                                                               good   good   bad    good                                             Mass     good   bad    good   bad                                              productivity                                                                            (pressureless                                                                         (hot press)                                                                           (pressureless                                                                         (hot press)                                      and producing                                                                           sintering)    sintering)                                              cost                                                                           __________________________________________________________________________

EXPERIMENT 3

SiC powder having the average particle diameter of not more than 5 μm, MoB₂ as an additive, and B (metallic boron) and C (carbon black) as a sintering assistant were mixed together with isopropyl alcohol at a mixing recipe shown in Table 6 by a wet type ball mill. The mixture was dried and granulated, which was preliminarily molded and then hydrostatically pressed at 3 tons/cm² to produce a square plate of 60×60×6 mm. The plate was heated up to 1,500° C. in vacuum, and then fired at 2,200° C. under 1 atm in argon atmosphere for 1 hour. In such a way, sintered bodies according to Examples of the present invention and Comparative Examples were obtained. Example 104-2 is a product obtained by HIP treatment of a sintered body in Example 104 at 2,000° C. under 2,000 atms.

With respect to the thus obtained sintered bodies, denseness, strengths at room temperature and at high temperatures (1,500° C.), and toughness were evaluated by the same methods as in Experiment 1. Further, a Mo compound in the sintered bodies was identified by an X ray diffraction method using CuKα. Results are shown in Table 6.

                                      TABLE 6(a)                                   __________________________________________________________________________                                                      Presence or                                          Four point                absence of                                 Sintering                                                                           Relative                                                                            bending strength          abnormal                              Additve                                                                             assistant                                                                           density of                                                                          (MPa)             Mo compound                                                                            grain growth                          (wt %)                                                                              (wt %)                                                                              sintered                                                                            room        K.sub.IC                                                                             in sintered                                                                            in sintered                           MoB.sub.2                                                                           B  C body (%)                                                                            temperature                                                                           1,500° C.                                                                    (MN/m.sup.3/2)                                                                       body    body                          __________________________________________________________________________     Comparative                                                                            0    1.0                                                                               4.0                                                                              92   450    360  2.3   not contained                                                                          present                       Example 101                                                                    Example 101                                                                            0.5  1.0                                                                               4.0                                                                              98   670    670  4.0   MoB.sub.2                                                                              no                            Example 102                                                                            5    1.0                                                                               4.0                                                                              98   690    670  4.4   "       "                             Example 103                                                                            10   1.0                                                                               4.0                                                                              98   720    700  4.8   "       "                             Example 104                                                                            30   1.0                                                                               4.0                                                                              98   740    730  5.2   "       "                             Example 104-2                                                                          30   1.0                                                                               4.0                                                                              not less                                                                            870    850  5.5   "       "                                               than 99                                                      Example 105                                                                            50   1.0                                                                               4.0                                                                              97   710    690  4.7   "       "                             Example 106                                                                            70   1.0                                                                               4.0                                                                              92   640    560  4.1   "       "                             Comparative                                                                            90   1.0                                                                               4.0                                                                              82   490    230  2.9   "       "                             Example 102                                                                    __________________________________________________________________________

                                      TABLE 6(b)                                   __________________________________________________________________________                                                      Presence or                                          Four point                absence of                                Sintering                                                                            Relative                                                                            bending strength          abnormal                             Additive                                                                            assistant                                                                            density of                                                                          (MPa)             Mo compound                                                                            grain growth                         (wt %)                                                                              (wt %)                                                                               Sintered                                                                            room        K.sub.IC                                                                             in sintered                                                                            in sintered                          MoB.sub.2                                                                           B  C  body (%)                                                                            temperature                                                                           1,500° C.                                                                    (MN/m.sup.3/2)                                                                       body    body                          __________________________________________________________________________     Comparative                                                                           30   0  4.0                                                                               74   310    260  --    "       no                            Example 103                                                                    Example 107                                                                           30   0.3                                                                               4.0                                                                               97   680    670  4.8   "       "                             Example 108                                                                           30   2  4.0                                                                               98   720    700  5.0   "       "                             Example 109                                                                           30   5  4.0                                                                               95   680    650  4.9   "       "                             Comparative                                                                           30   10 4.0                                                                               89   490    400  3.9   "       "                             Example 104                                                                    Comparative                                                                           30   1.0                                                                               0  62   250    230  --    "       "                             Example 105                                                                    Example 110                                                                           30   1.0                                                                               2  97   740    720  5.1   "       "                             Comparative                                                                           30   1.0                                                                               10 85   500    490  3.4   "       "                             Example 106                                                                    __________________________________________________________________________

As is evident from Table 6, Examples 101 to 110 satisfying the composition range according to the present invention had higher denseness and more excellent strength at high temperatures as compared with Comparative Examples 101 to 106, and had higher K_(IC) values as compared with K_(IC) (2-3) in the conventional case of SiC containing no MoB₂ as well as higher toughness. In Comparative Example 101 added with no MoB₂, abnormal grain growth of SiC was recognized, but no such abnormal grain growth was recognized in the other Examples. In addition, Example 104 having undergone the HIP treatment after the firing had more excellent characteristics in all respects.

Furthermore, it was confirmed that ,the Mo compound in the sintered bodies was MoB₂, as indicated in JCPDS card No. 6-682.

FIG. 3 shows an X ray diffraction pattern of Example 3 in Table 3 by using CuKα.

As a result of the chemical analysis of the to 0.08 wt% of inevitable metallic impurities (Al, Fe, Ca, Mg, Ti, Mn, etc.) contained in the SiC starting material and the additives were detected.

EXPERIMENT 4

Instead of MoB₂ as the indispensable ingredient of the present invention, a boride other than MoB₂, a carbide or an oxide of Mo, or Mo was added, followed by milling and mixing, and sintered bodies were obtained by the same method as in Experiment 3.

Then, with respect to the thus obtained sintered bodies, the relative density and a Mo compound were determined by the same methods as in Experiment 3. Results are shown in Table 7.

                  TABLE 7                                                          ______________________________________                                                                Relative                                                              Sintering                                                                               density                                                               assistant                                                                               of                                                             Additive                                                                              (wt %)   Sintered Mo compound in                                        (wt %) B      C     body (%)                                                                              sintered body                                ______________________________________                                         Comparative                                                                             Mo:30    1.0    4.0 58     MoC, Mo.sub.2 BC                           Example 107                                                                    Comparative                                                                             Mo.sub.2 C:30                                                                           1.0    4.0 57     MoC, Mo.sub.2 BC                           Example 108                                                                    Comparative                                                                             MoC:30   1.0    4.0 57     MoC, Mo.sub.2 BC                           Example 109                                                                    Comparative                                                                             Mo.sub.2 B:30                                                                           1.0    4.0 62     MoB                                        Example 110                                                                    Comparative                                                                             MoB:30   1.0    4.0 63     MoB.sub.2, MoB                             Example 111                                                                    Comparative                                                                             MoO.sub.3 :30                                                                           1.0    4.0 55     MoC, Mo.sub.2 BC                           Example 112                                                                    Comparative                                                                             Mo:30    5      4.0 61     MoB, MoB.sub.2                             Example 113                                                                    Example 111                                                                             Mo:30    10     4.0 98     MoB.sub.2                                  Comparative                                                                             MoC:30   5      4.0 62     MoB, MoB.sub.2                             Example 114                                                                    Example 112                                                                             MoC:30   10     4.0 97     MoB.sub.2                                  ______________________________________                                    

As is evident from the results in Table 7, in the case of Comparative Examples 107 to 112 using an additive other than MoB₂, high relative density, i.e., densification of the sintered bodies could not be achieved if B was contained in a conventionally known amount. Further, in Comparative Examples 13 and 14 having an increased amount of B, when B was not converted to MoB₂ in the sintered body, high relative density could not be achieved.

In the case of Examples 111 and 112 in which the amount of B was further increased up to an addition amount being more than two times in mole % than that of Mo (Mo: 30 wt%, B: 6.8 wt%), MoB₂ was formed in the molding during the firing initial stage, so that densification could be attained as in the case with Example 101 in which Mo was added in the form of MoB₂.

As is evident from the foregoing detailed explanation, according to the SrC complex sintered bodies and the producing process thereof in the present invention, by incorporating a specified amount of MoB₂ into the B-C based SiC, sintering can be effected at high temperatures and toughness which is poor in the B-C based SiC can be improved while strength at high temperatures is maintained. Further, SiC complex sintered bodies having high strength at high temperature and high toughness can be obtained by pressureless sintering.

In Table 8, characteristics of the SiC complex sintered bodies according to the present invention are shown in comparison with those of a prior art technique.

                                      TABLE 8                                      __________________________________________________________________________                     Al based                                                                              B--C based                                                                            Al based                                                  SiC--MoB.sub.2                                                                        sintering                                                                             sintering                                                                             sintering                                                 according to                                                                          assistant +                                                                           assistant +                                                                           assistant +                                               the invention                                                                         SiC--MoB.sub.2                                                                        SiC    SiC                                              __________________________________________________________________________     High temper-                                                                            good   bad    good   bad                                              ature charac-                                                                  teristic                                                                       Toughness                                                                               good   good   bad    good                                             Mass     good   bad    good   bad                                              productivity                                                                            (pressureless                                                                         (hot press)                                                                           (pressureless                                                                         (hot press)                                      and producing                                                                           sintering)    sintering)                                              cost                                                                           __________________________________________________________________________ 

What is claimed is:
 1. A pressureless sintered silicon carbide complex sintered body consisting essentially of 20 to 99 wt% of SiC, 70 to 5 wt% of W₂ B₅, and 0.5 to 5 wt% of at least one material selected from the group consisting of boron, carbon, and boron carbide.
 2. A pressureless sintered silicon carbide complex sintered body according to claim 1, wherein W₂ B₅ is present in an amount from 60 to 10 wt%.
 3. A pressureless sintered silicon carbide complex sintered body according to claim 1, wherein SiC is present in amount from 30 to 95 wt %.
 4. A pressureless sintered silicon carbide complex sintered body according to claim 1, wherein SiC is present in amount from 40 to 90 wt %.
 5. A pressureless sintered silicon carbide complex sintered body consisting essentially of 20 to 99 wt % of SiC, 70 to 0.5 wt % of MoB₂, and 0.5 to 5 wt % of at least one material selected from the group consisting of boron, carbon, and boron carbide, wherein said sintered body consists crystalline phases of MoB₂ and SiC.
 6. A pressureless sintered silicon carbide complex sintered body according to claim 5, wherein MoB₂ is present in an amount from 60 to 0.5 wt%.
 7. A pressureless sintered silicon carbide complex sintered body according to claim 5, wherein SiC is present in amount from 30 to 95 wt %.
 8. A pressureless sintered silicon carbide complex sintered body according to claim 5, wherein SiC is present in an amount from 40 to 90 wt %.
 9. A process for producing pressureless sintered silicon carbide complex sintered bodies having a relative density of at least 92%, comprising the steps of:mixing powders to form a mixture consisting essentially of 20 to 99 wt% of SiC powder having an average particle diameter of not greater than 5 microns, 80 to 0.5 wt%, when calculated as W₂ B₅, of W₂ B₅, or a W₂ B₅ -forming compound, 0.1 to 5 wt%, when calculated as boron, of boron or a compound containing boron, and 0.1 to 5 wt%, when calculated as carbon, of carbon or an organic compound producing carbon; molding the mixture; and firing the molding in a temperature range from 1,900° to 2,500° C. in vacuum or in an inert atmosphere to form said silicon carbide complex sintered bodies.
 10. A process according to claim 9, further comprising subjecting the SiC complex sintered bodies to a capsule HIP or a capsule-free HIP treatment.
 11. A process for producing pressureless sintered silicon carbide complex sintered bodies, comprising the steps of:mixing powders to form a mixture consisting essentially of 20 to 99 wt % of SiC powder having an average particle diameter of not greater than 5 microns, 80 to 0.5 wt %, when calculated as MoB₂, of MoB₂, or a MoB₂ -forming compound, 0.1-5 wt %, when calculated as boron, of boron or a compound containing boron, and 0.1 to 5 wt %, when calculated as carbon, of carbon or an organic compound producing carbon; molding the mixture; and firing the molding in a temperature range from 1,900° to 2,500° C. in vacuum or in an inert atmosphere to form said silicon carbide complex sintered bodies; wherein said sintered bodies consist of crystalline phases of MoB₂ and Sic.
 12. A process according to claim 11, further comprising subjecting the SiC complex sintered bodies to a capsule HIP or a capsule free HIP treatment. 